I'm running a CPU intensive command on Mavericks (Opencv_traincasscade). Under Mountain Lion the process used to use all 8 cores all the way though. However after upgrading it drops to 1 core after a short time.

Thanks you for the answer, I will definitely give that a go. However, there are 2 reasons why I think it might not be the case. Firstly, the CPU did hit 800% for a few mins when it started, which suggested that OpenCV is capable of using all cores. Secondly I never encountered this problem on Mountain lion (however, I cant be 100% sure if it was the same build). The -mode parameter is related to rotation of feature detection not cores I believe.
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RobertMay 26 '14 at 14:33

Actually I take that back! You were right :D I'm going to answer for completeness.
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RobertMay 26 '14 at 16:49